Tower Mounted Amplifier

Tower Mounted Amplifier or Mast Head Amplifier is a Low-noise amplifier (LNA) mounted as close as practical to the antenna.

Utilizing maximum coverage is one of the key objectives of designing and optimizing a Radio / Wireless network. Achieving maximum coverage is often as easy as boosting the uplink signal (Received from Mobile phone) from the network users phone at the base station. The uplink limitation is always the case because of lower transmit power used by the mobile station. Function of TMA or MHA remains same for GSM/UMTS/PCS etc.

Radio Coverage in UMTS (WCDMA) networks is largely considered to be uplink limited in low traffic conditions. The basis for this is that the base station has typically 10-40 W (40-46 dBm) output power available, while the mobile unit has 0.125-0.250 W (21-24 dBm). This means that in low traffic situations, the uplink is the limiting link, while in high traffic situations, downlink becomes the limiting link. For these reasons infrastructure vendors strongly recommend using TMAs with their base station solutions.

Appropriately installed Low Noise Amplifiers (LNAs) in the uplink will significantly improve receiver system sensitivity when installed as close as possible to the receive antenna, particularly where cable losses are significant. LNAs located in this way are referred to as Tower Mounted Amplifiers (TMAs).

Installing TMAs in a CDMA system (like UMTS WCDMA) is not as straightforward as in GSM. In GSM it is easy to determine when to install TMAs using statistical data like the link balance report, where it shows how much imbalance you have between uplink and downlink.

TMA’s are sometime also called MHA (Mast Head Amp ) as they are installed very close to Receive path of Antenna as shown above.

Base Station Receiver Sensitivity

In WCDMA receiver sensitivity is a combination of four fundamental factors within a network:

• Thermal Noise Density – is a measure of the radio signal noise in nature. In the bandwidth of a WCDMA carrier this is -174 dBm/Hz at 290 K.

• Eb/No – it is the energy per bit to total noise spectrum density ratio. This is the signal to (interference + noise) ratio after the despreading process. It contains the processing gain. The values are vendor-specific and based on specific service.

• Noise Figure (NF) – is a measure of extra noise caused by the receiver circuitry. For the WCDMA Node B this is typically 3 dB.

• User data rate – the data rate of the type of service (e.g. 12.2 AMR, 384 kbps PS)

The maximum sensitivity of a WCDMA receiver channel is (e.g. using 12.2 kbps AMR):

Sensitivity	= Thermal Noise Density + Noise Figure + Eb/no + 10log (user data rate)
Sensitivity	= -174 + 3 + 8 + 10log (12.2 kbps)
Sensitivity	= -121 dBm

Considering additional losses coming from feeder and connectors, typically around 4 dB, the sensitivity can go down to -117 dBm.

Being a function of the WCDMA carrier bandwidth and circuit design, Thermal Noise Density and Eb/No are fixed as far as the network operator is concerned (Eb/No is normally set by vendor as specific to their equipment). The only component affecting receiver sensitivity that may be improved by the operator is the system Noise Figure

Table of different sensitivity levels

Vendor	Service Type	Bit Rate (kbps)	Eb/No (dB)	Node B NF (dB)	UL Sensitivity with no LNA (dBm)	Feeder Loss (dB)	System NF with no TMA (dB)	TMA gain (dB)	TMA NF (dB)	System NF with TMA (dB)	UL Sensitivity with TMA (dBm)
Ericsson	AMR	12.2	4.8	3	-121.34	4	7	12	2	2.6	-125.69
Ericsson	CS	64	2.8	3	-116.14	4	7	12	2	2.6	-120.49
Ericsson	PS	64	3.4	3	-115.54	4	7	12	2	2.6	-119.89
Ericsson	PS	128	3	3	-112.93	4	7	12	2	2.6	-117.28
Ericsson	PS	384	2.8	3	-108.36	4	7	12	2	2.6	-112.71

Receiver sensitivity of various services with and without TMA

Benefits

A TMA can be used to reduce the system noise figure (NF) and therefore increase sensitivity. As previously mentioned a TMA is a LNA mounted as close as practical to the antenna. In this way, the cable losses are negligible and do not significantly affect system noise figure. This reduction in uplink system noise figure equates to an enhanced uplink providing larger coverage footprint, improvement in “in-building” performance and enhanced UE talk time. This is particularly apparent in the initial stages of a newly-launched network (under zero or minimal load).

System NF is calculated as follows:

System Noise Figure (NFs)	= 10log (Fs)
System Noise Factor (Fs)	= F1 + [(F2 – 1)/G1]

Where:

NFs = System Noise Figure (in dB)
Fs = System Noise Factor (multiplier not dB)
F1 = Noise factor of the TMA (multiplier not dB)
F2 = Noise factor of the base station receiver (includes system losses, e.g. feeder, connector losses)
G1 = Gain of the TMA (multiplier not dB)

In theory cascading TMAs will reduce the NF toward zero. A NF of between 1.5 and 2 dB is the practical limit of the base station system. The choice of TMA gain is as crucial as its NF. Typically the range is 8 dB to 16 dB. Less than 8 dB gain and the NF improvement reduces significantly and with more than 16 dB you will only be amplifying the noise floor and incurring excessive base station dynamic range compression.

The following table gives an example of the typical improvements that are possible using TMAs with different gain

Base Station Configuration 12 dB Gain TMA (NF = 2 dB) 16 dB Gain TMA (NF = 2 dB) BS NF = 3 dB Feeder Loss = 4 dB NF = 3 + 4 = 7 dB NF = 2.6 dB (improvement of 4.4 dB) NF = 2.3 dB (improvement of 4.7 dB)

With a 12 dB gain LNA, one can expect that the UL sensitivity can be improved from -117 dBm to -122 dBm (close to 5 dB improvement). Typically a low noise TMA with a gain of 12 dB would be used to achieve this result.

This improvement in uplink sensitivity simply allows the base station to work with farther mobiles. With growing amount of traffic the improvement actually decreases, as the link budget will be more and more DL limited. Due to increased uplink sensitivity the number of users in uplink which can be served increases too. One of the reason can be that if more users can be served in uplink, the transmit powers in downlink increase due to possible more SHO connections and thus reducing remaining downlink capacity (downlink power). Therefore, the question how much of the uplink coverage improvement can be utilized in the downlink direction depends on the current downlink load.

The effect of a TMA in the downlink is the additional TMA insertion loss (normally it is less than 1 dB).

External links

• TMA test equipment
• Mast-Head Amplifier Interference

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